Back firing

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Restomodcougar
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Re: Back firing

Post by Restomodcougar » 09 Apr 2018, 20:57

Check leads, could have a cracked spark plug from putting them in. Did you rotate the crank at all before putting the cam back in?

superbond
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Re: Back firing

Post by superbond » 10 Apr 2018, 04:31

I’ve driven this car for around 15 mile this no issues. After it’s been sitting for a month or two these back fires seem to be happening. I will check the compression this weekend and go from there. The push rods are not adjustable. If I did tighten the rocker arms down to long. Would it do it from the first start up?

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Re: Back firing

Post by superbond » 10 Apr 2018, 18:46

Yes I did rotate the crack because I removed the pistons to make sure the rings weren’t lined up and everything was ok.

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Restomodcougar
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Re: Back firing

Post by Restomodcougar » 11 Apr 2018, 19:40

superbond wrote:
10 Apr 2018, 18:46
Yes I did rotate the crack because I removed the pistons to make sure the rings weren’t lined up and everything was ok.
I once had a cam out 180 degrees once as I only rotated it once to get the piston up at a top dead center. One turn is compression and the other is exhaust. I am just thinking this because I did something similar when I rebuilt and installed a Lumpy cam in a 72' 302 back when I was in my 20's and I could get it to start but it would misfire bad and a loud backfire out the carb.

Man that was a long time ago!

Anyways, I had to take apart the front of the engine and take the timing chain off and rotate the cam the 180 degrees from where it was and put it all back together again and use a cam wheel.

Now if I am totally off base because you said you drove it 15 miles and you installed a new cam, the specs could require it to run more at a + or - TDC compared to the stock one.

It ran fine when cool but misfired when hot or under load for those 15 miles???

My only other thoughts are a short somewhere like distributor cap, broken plug, pinhole or crack in a spark plug wire, a leaking boot or bad contacts or coil. These can also not show signs when all items are cold and when the material heats up causes expansion of the plastics, etc and could cause something to arc. I had a yellow top coil with a hairline crack that I couldn't see with my eyes do this once.

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1969XR7Vert
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Re: Back firing

Post by 1969XR7Vert » 12 Apr 2018, 11:27

superbond wrote:
09 Apr 2018, 09:19
I did put the used cam in and lifters in the exact spot. The only thing I changed was I put new push rods in. They were the same length and were straight.
It just seems like the issue is getting worse every time I start it up.
I torqued the rocker arms to 11-14 ft pounds with lock tight.
I’m going to switch to my old points and see if that helps
Did you use a zinc (ZDDP) additive in (every) oil change? Not just for break in, every 5 quarts must have zinc either by additive or by being "special" oil that has it already. If the answer is no, do the check I mentioned in my first post in this thread.
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superbond
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Re: Back firing

Post by superbond » 12 Apr 2018, 12:35

I did put additives in and I’m thinking it could be me pertonix ignition I installed on my stock distributor

DaveK
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Re: Back firing

Post by DaveK » 12 Apr 2018, 13:08

superbond wrote:
12 Apr 2018, 12:35
I did put additives in and I’m thinking it could be me pertonix ignition I installed on my stock distributor
If you're not feeding the Pertronix 12V, it won't work.

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xr7g428
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Re: Back firing

Post by xr7g428 » 12 Apr 2018, 13:25

My problem is the motor is back firing out the driver side exhaust. I don’t have crossover on my exhaust. So you can hear a popping sound coming from the driver side exhaust. Pop pop pop pop! It seem to be getting worse.
I torqued the rocker arms to 11-14 ft pounds with lock tight.
From what you are telling us it sounds like the exhaust valve is slightly open on one cylinder on the drivers side. It sounds like you hear the pop every time the cylinder fires. You may be able to locate the cylinder by disconnecting the plug wire on one cylinder at a time. This may work, but sometimes when the fuel air mix hits the hot exhaust it may explode (in the exhaust manifold = backfire) even without a spark.

Also, you don't set preload with a torque wrench. To adjust preload on hydraulic lifter camshafts with adjustable rocker arms adjust the valve to zero lash (I spin the push rod with my fingers and when it feels tight and resists turning, that is zero lash) with no preload, then 1/2 to 1 turn more. It sounds like you have adjusted one lifter too tight. Another less likely possibility is that you have a stuck valve. If that is the case you should be hearing some racket from the lifter.

About the only timing related issue is if you have mixed the spark plug wires and you have a plug firing when the exhaust vavle is partially open. It should run like poop, missing badly on two cylinders if that is the case.
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Legendary Cougar Magazine
Currently in the Cat House
'67 XR7 GT 390 4 speed, AC, AM FM, Lime Frost Green
'68 XR7-G 428CJ C6, Tilt-Away, AM, Black Cherry
'68 XR7-G 390 4 speed, Sunroof, Cardinal Red
'68 XR7 GT-E 427 C6 AM Cardinal Red
'68 XR7 resto mod 351W, soon to be AOD, Black Cherry

superbond
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Re: Back firing

Post by superbond » 12 Apr 2018, 14:45

Restomodcougar wrote:
11 Apr 2018, 19:40
superbond wrote:
10 Apr 2018, 18:46
Yes I did rotate the crack because I removed the pistons to make sure the rings weren’t lined up and everything was ok.
I once had a cam out 180 degrees once as I only rotated it once to get the piston up at a top dead center. One turn is compression and the other is exhaust. I am just thinking this because I did something similar when I rebuilt and installed a Lumpy cam in a 72' 302 back when I was in my 20's and I could get it to start but it would misfire bad and a loud backfire out the carb.

Man that was a long time ago!

Anyways, I had to take apart the front of the engine and take the timing chain off and rotate the cam the 180 degrees from where it was and put it all back together again and use a cam wheel.

Now if I am totally off base because you said you drove it 15 miles and you installed a new cam, the specs could require it to run more at a + or - TDC compared to the stock one.

It ran fine when cool but misfired when hot or under load for those 15 miles???

My only other thoughts are a short somewhere like distributor cap, broken plug, pinhole or crack in a spark plug wire, a leaking boot or bad contacts or coil. These can also not show signs when all items are cold and when the material heats up causes expansion of the plastics, etc and could cause something to arc. I had a yellow top coil with a hairline crack that I couldn't see with my eyes do this once.

How can you do something like that when you have to line up the timing chain gears? Am I missing something? Hmmmmmmm

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xr7g428
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Re: Back firing

Post by xr7g428 » 12 Apr 2018, 16:27

The cam rotates once for every two turns of the crank shaft. But that is not your problem. If it were 180 degrees out it would run so bad that it wouldn't get out of your garage. If you did set the valve lash by adjusting the rocker arm nuts with a torque wrench you need to go back and redo that as torque has nothing to do with the settings.
Bill Basore, Editor / Publisher
Legendary Cougar Magazine
Currently in the Cat House
'67 XR7 GT 390 4 speed, AC, AM FM, Lime Frost Green
'68 XR7-G 428CJ C6, Tilt-Away, AM, Black Cherry
'68 XR7-G 390 4 speed, Sunroof, Cardinal Red
'68 XR7 GT-E 427 C6 AM Cardinal Red
'68 XR7 resto mod 351W, soon to be AOD, Black Cherry

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